In recent years, according to widespread of electrical equipment such as motors, an abnormal operation of the electrical equipment is more and more likely to lead a serious accident, and it is thus required to reduce risks of the accident as much as possible. International standards are defined in order to reduce the risks to be within an allowable range. IEC61508 is defined as a standard regarding electrical equipment, and IEC61800-5-2 defined as a standard regarding driving equipment such as motors.
A Safe Torque-Off function (STO) is known as a safety function defined in the IEC61800-5-2. When externally receiving an interruption instruction, the Safe Torque-Off function interrupts power supply to a motor to stop an operation of the motor. To be able to surely interrupt power supply when required is an important function in terms of safety, which is not limited to the case of the motor. To that end, it is important to periodically “diagnose” whether or not a power interrupting device for interrupting power supply normally operates.
Patent Literature 1 discloses a technique that monitors whether or not there is an abnormality in an interrupting device in an operating condition. More specifically, the interrupting device uses gate signals to interrupt PWM signals supplied from a control device to a bridge circuit. A monitoring device generates a test signal to diagnose the interrupting device. A switching circuit switches the test signal and an external interruption signal. A delay circuit permits passage of an output signal from the switching circuit after elapse of a set time from a change point of the output signal. An input signal to the delay circuit is fed back, as a feedback signal, to the monitoring device. The monitoring device outputs the test signal to the interrupting device at an interval shorter than the delay time and determines whether or not the feedback signal coincides with the test signal.